In certain industrial applications, such as for example hydromechanical fuel controls, hydraulic servovalves and other fluidicly actuated devices, it is necessary to deliver fluid to the device, be it fuel, hydraulic fluid or actuation fluid, at a controlled rate and at a regulated pressure. The mass flow rate of a constant density fluid through a conduit is a function of the flow area thereof and the pressure drop therethrough. Thus, conventional flow control valves suitable for metering fluid flow and regulating fluid pressure generally incorporate both a flow throttling element and a pressure regulating element, which are independently adjustable. Typically, the pressure regulating valve operates to maintain a constant pressure drop across the throttling element, thereby permitting the rate of flow through the valve to be controlled by varying the flow area by adjustment of the throttling element. The pressure at which the fluid is delivered from the valve is less than the pressure of the fluid supplied to the valve by an amount equal to the pressure drop across the throttling element, which is maintained at a constant by the pressure regulating element. While satisfactory for use with constant density fluids, such flow control devices can not maintain a constant pressure drop if the temperature, and consequently density, of the fluid varies substantially over the operational range of the device.
A flow control device which is capable of maintaining a constant drop in response to varying fluid temperature, and therefore varying fluid density, is described in commonly assigned U.S. Pat. No. 4,440,191 to Hansen. As disclosed therein, the flow device comprises an axially positionable flow throttling element disposed within the valve cavity and a pressure regulating element which is coaxially received within the throttling element and is independently axially translatable with respect thereto. The flow throttling element is selectively positionable by application of a servofluid to vary the flow area through the device. The pressure regulating element is spring biased against supply pressure and independently positionable relative to the throttling element by applying regulated pressure, i.e. supply pressure reduced by the pressure drop, thereto in opposition to the supply pressure. Concave-convex bimetallic discs are disposed so as to vary the spring bias force upon the pressure regulating element in response to changes in the fluid temperature. While functional to vary the pressure drop by a few psi in response to small changes in fluid temperature, such a flow control valve can not function to give large changes in pressure drop, such as 50 to 200 psi, in response to greater fluid temperature variations on the order of 20 to 100 degrees Fahrenheit.